Chemical mechanical polishing (CMP) processes are commonly used in the manufacture of integrated circuits to planarize wafer surfaces. As shown in the prior art cross-sectional schematic drawing of FIG. 1, typical CMP systems include a semi-porous polishing pad 17 mounted on the upper surface of a planar platen 16. The polishing pad is wetted with a chemically reactive, abrasive slurry from a supply tube 18. Commonly, the platen is relatively large in comparison to a wafer 12 to be planarized and is rotated during the polishing process. Wafer 12 is held by means of a wafer carrier 11, which typically is capable of transverse movement 15 and also rotational movement about a shaft 14. The rotational and transverse movement of the wafer with respect to the polishing pad facilitates uniform CMP etch rates across the wafer surface.
There are many variables that affect the ability of a specific CMP process to planarize a wafer surface. These include the pressure between polishing pad 17 and wafer 12, the hardness of polishing pad 17, the slurry composition, and the relative motion between the platen and the wafer (e.g., platen and wafer rotation rates). One important variable of a CMP process is the rate at which fresh polishing slurry is supplied. During a CMP process, chemical components of the slurry are continuously consumed by the polishing process. Waste by products of the polishing process are also generated. The nature of the deleterious waste products will depend upon the particular polishing process, but may include reacted chemical by-products of the polishing process, degraded polishing pad components, or particulates from the abrasive component of the slurry. The chemical and mechanical aspects of the polishing process may change if the active components of the polishing slurry become depleted or if deleterious waste products build up. A constant flow of fresh slurry to the platen is thus desirable to replenish the active components of the slurry and to flush out deleterious waste products.
Fresh slurry is typically supplied to wafer 12 on a continuous basis, such as by dripping a continuous stream of slurry from supply tube 18 onto a portion of pad 17. In addition to refreshing the reacted or depleted slurry, slurry must also be supplied because centrifugal force tends to fling slurry off of the edge of the platen as the platen rotates. As shown in the prior art cross-sectional drawing of FIG. 2, polishing pad 17 rotates at an angular velocity .omega..sub.p. The equivalent linear speed (L) of the polishing pad at a radius, r, from a central axis 0, is .omega..sub.p r. Also, as shown in FIG. 2 the wafer 12 may also be rotated about its axis at an angular velocity .omega..sub.w.
At high platen rotation rates, a substantial flow of fresh slurry onto the polishing pad 17 from the supply tube 18 may be required to compensate for slurry flung off from the edges of the platen. Also, at high platen rotation rates, substantially larger quantities of slurry are flung from the platen and at a higher velocity. This increases the difficulty of containing slurry chemicals and particulates proximate to the polishing system. Additionally, the increased slurry consumption increases the cost of the polishing process. Another problem associated with high platen rotation rates is that the polishing pad may become unevenly wetted. The edge regions of the pad will tend to become substantially wetter than the center most pad regions because of the effect of centrifugal force. This is highly undesirable as it may result in non-uniform polishing across the polishing pad.
One attempted solution to these problems is flood polishing. In flood polishing schemes, dams are erected around the circumference of the platen to hold in the polishing slurry. Flooding the platen with a deep pool of slurry facilitates wetting the entire pad. The dam acts to retain the slurry from being flung off of the platen such that typically no additional slurry is dripped onto the platen during the polishing process. However, such flood polishing schemes have several limitations. First, in common flood polishing schemes, there is no simple technique to continuously refresh consumed slurry components and to flush out deleterious waste products. The level of polishing slurry is typically chosen to flood the entire platen with approximately a quarter inch (6.35 mm) of slurry in order to provide a reservoir of polishing components to supply all polishing needs. Additionally, the slurry reservoir must be large enough that waste products do not build up to deleterious levels. Second, in conventional flood polishing methods, there is no simple way to continuously adjust the slurry depth as a function of platen rotation rate. This is undesirable because fixing the slurry depth at one initial level will tend to limit the variations in platen rotation rate that are feasible during the polishing process. For example, because flood polishing uses a deep pool of slurry, it may suffer from undesirable hydroplaning at high platen rotation rates. In the most general case, the mechanical energy imparted by the polishing pad to the wafer will depend both on platen rotation rate and upon the slurry depth. Fixing the slurry depth at a constant level thus limits the ability of a process engineer to control the mechanical component of a chemical mechanical polishing process.
What is desired is an apparatus and method to increase control of the flow of polishing slurry on a rotating platen used in a chemical mechanical polishing process.